At present in the USSR submersible nozzles of quartz glass are widely used in machines for continuous casting of steel. A high heat resistance is an advantage of quartz material.The wise use of continuous casting of steel in the whole World and the ever-increasing use of series casting, especially of mold steels, had led to a significant increase in the use of amorphous silica parts [i, 2].In the USSR submersible quartz nozzles to TU 14-8-231-77 are produced by the Podoisk Refractory Part Plant.The parts are made by slip casting of a suspension with a density of up to 1.80 g/cm 3 in gypsum molds.The formed green part is dried and fired in tunnel kilns at I150~The possibility of crystallization of the material in high-temperature firing is a significant disadvantage of this method.The presence of cristobalite reduces the heat resistance of the parts as the result of a sharp change in the linear dimensions in the polymorphic $ § ~-transformation of cristobalite.In connection with the above there is significant interest in the method of unfired quartz parts [3] in which the material is hardened by hydrothermal treatment.Such a treatment eliminates the formation of cristobalite and, in addition, provides a reduction in the consumption of thermal energy in production of the parts.The purpose of this work was the development and comparison of different variations of the method for submersible quartz nozzles for continuous casting machines.The work was done in collaboration with Podolsk Refractory Part Plants.In the production of unfired quartz refractories the properties of the original highly concentrated suspension have a special influence.During wet grinding of the paticles of fused quartz complex processes causing not only obtaining of the specified fineness but also mechanicochemical activation of the dispersed phase occur. The main problem consists of creation of a cast system revealing binding properties.The latter are determined by a combination of characteristics (volume share of the solid phase CV, degree of dispersion, pH, viscosity, etc.) formed directly in grinding [4].The rate of grinding, evaluated, for example, by the content in the system of fine (<5 pm) or coarse (>50 or >63 pm) fractions, is determined by many factors.The most general of them, which are the result of the type of grinder and the composition of the charge, are the effective density of the grinding bodies (APg.b is the difference in the density of the grinding bodies and the suspension), the active zone of grinding (~ is the ratio of the volume of voids between the grinding bodies to the volume of suspension being ground), and the size (diameter) of the grinder.It is known that in the highly concentrated suspension method there are two methods of wet grinding, with single-step and with portion (step-by-step) charging of the material. The first method is used in grinding comparatively coarse porosity-free materials such as quartz glass or fused quartz.In wet grinding of such a material in a ball mill the processes of crushing and fine gr...

The processes of the formation of a solid material in VKVS (highly concentrated ceramic bonding suspensions) technology are characterized in the light of a number of changes, accompanied by alterations in the nature of the bond between the particles [1 ]. The primary structure formation of VKVS is accomplished during their partial dewatering, for example, on account of the porous mold, the filler, etc. The green article obtained from VKVS of acid or acidamphoteric composition, as a rule, is characterized by a rather high mechanical strength and elastobrittle destruction. Despite the fact that, both for the original casting in which all the pores are filled with liquid (volume proportion of kinetically bonded liquid Cws), and also for the casting after partial drying (right upto a moisture content of 0.5-I%), there is a characteristic lack of water resistance [2]. The water resistance, taken as the criterion for assessing the bonding properties of VKVS [3] is attained after drying the material to a moisture content of less than 0.1-0.2% [2]. The latter is explained mainly by the formation in the system of siloxane bonds. The probability of their formation between the microparticles of colloidal particles and the active colloidal covering of the grains of mineral constituent of VKVS in the undried material is low.Previously [2] using high-precision indicators for linear displacement it was shown for the first time that during the drying out of castings of siliceous composition, after a certain shrinkage, there is an expansion. It was postulated that it is due to polymerization of the interstitial silicic acid.Starting from the assumption about the interaction of the expansion of the casting and the water resistance and strength after drying, in the present article we deal with the problem of investigating the mechanism and features of the structure formation during drying out, bearing in mind the influence of such parameters as the type, dispersion and deduced concentration of the colloidal component cPrkk, and the critical concentration C v cr of the original VKVS.The linear changes in the castings in relation to their moisture content were studied on equipment shown in Fig. 1. The use of the induction sensor was useful for registering the change in the linear dimensions of the casting with an error of less than _+ 1 #m.The original VKVS (see Table 1) were studied in the form of suspensions of fused quartz, ordinary chamotte (27.2% A12Os), and mixed suspensions in the system ordinary ehamotte--quartz sand in the ratio 85:15 [4]. The suspensions were obtained by wet milling using the known technology, and stabilization [3]. The specimens measuring 10 • 10 • 8 mm were shaped in gypsum molds. Directly after the completion of the casting up of the specimen the casts were extracted, and then one of them was established on the equipment for measuring the linear dimensions, and using two parallel measurements we established continuous weighing in order to construct the relationship ACre 1 = f(W).Taking into account the ...

Until the present ime, slip casting in gypsum molds has been the preferred method for shaping quartz ceramics [I, 2].When using highly concentrated binder suspensions (HCBS) that are based on quartz glass or fused quartz, one can attain low porosity levels in the castings [3,4]. This is extremely important from the standpoint of technology (in particular, during the production of unfired materials [4]). However, this method has significant shortcomings such as slow rate of shaping and rapid wear of the gypsum molds.In view of this, the method of centrifugal forming of castings from the same HCBS that are often used for slip casting is of considerable practical interest. This method makes it possible to eliminate the aforementioned shortcomings of slip casting.It is mainly used for shaping solids of revolution such as the nozzles of the steel casting ladles (downtubes), heat insulating pipes, and roller casings of the "Tandem" type furnaces. However, when the process is perfected, products having inserts (lateral parts) can also be molded.As concerns the shaping of ceramics and refractories, the process of centrifugal casting has been used rarely up to the present time and, therefore, virtually no studies have been carried out on these aspects. In view of this, we examined the general regularities of the process of centrifugal casting and the effect of the main technological factors on it.The highly concentrated (C V = 0.70-0.75) fused quartz suspensions that are used foe obtaining unfired quartz ceramics [4,5] formed the basic experimental materials. Experimental specimens were obtained in a metallic (porefree) mold using the previously described installation [5]. The experiments conducted using HCBS of the mullite, chamotte, and other systems showed that the main regularities established during the study of the HCBS of fused quartz are generally valid for the centrifugal tasting process of all the investigated systems.Just as in the process of centrifugal forming of ferro-concretes [6,7], when a mold containing a uniformly distributed suspension is rotated around a fixed axis at a predetermined speed, a centrifugal pressure Pc is developed in the molding system, and under the action of this compressive pressure the dispersing medium is squeezed out from the suspension along with the finest particles suspended in it. During this process, the larger particles of the solid phase are simultaneously drawn closer to each other.The dispersing medium of HCBS receives the centrifugal pressure directly. In view of this, the excess hydrostatic pressure increases and leads to filtration of the liquid and simultaneous densification of the solid phase of HCBS. The nmnber of revolutions of the mold per unit time n (or the circumferential or the linear velocity ~ derived from it) forms the most important parameter of the process.The compressive (compacting) pressure is distributed nonuniformly across the wall thickness of the product (just as the hydrostatic pressure). Its maxim~n and the minimum values are obtained at the inte...

In a recent study [I], we established the main regularities of the centrifugal casting process and some of the technological parameters required for casting the highly concentrated binder suspensions (HCBS) of fused quartz.This paper deals with a specific study of the effect of the technological and the theological properties of HCBS of fused quartz on the porosity Pcsg and the strength of the castings based on them. Some aspects of the strengthening process of-the castings used for obtaining unfired ceramics [2] are also examined.During centrifugal casting, besides the concentration of HCBS and the coefficient of excess solid phase Kex s [i, 2], the circumferential velocity ~ has a significant effect on the parameter Pcsg (Fig. I, region i). In this region, the range of variation of Pcsg at different values of ~ is determined by various technological factors (for example, Kexs). However, a regular increase of the porosity of the castings is observed with increasing of the casting process. On increasing ~ from 3 up to 20 m/sec, Pcsg increases from 11-14 up to 19-22%. The corresponding values of the apparent density of the castings are found to decrease from 1.86-1.89 up to 1.71-1.78 g/cm s. The aforementioned regularity owes to the rheologicai properties of the HGBS (in particular, their dilatancy) and to the excessive layering of the castings (that, in turn, hinders dense packing of particles) at high values of ~ of the process.It is known [3,4] that the increased porosity of the slip-cast quartz ceramics in the limits indicated in Fig. 1 (see region i) is accompanied by a significant loss of their strength.During centrifugal casting, abn d of the castings in the as-dried condition (region 2) as well as in the strengthened condition (region 3) remain unaltered. The aforementioned anomaly possibly owes to the structure (in particular, the specific features of the grain size distribution in the castings) and requires a special investigation.Just as in the case of slip casting, the pH value of the original HCBS (that, in turn, determines the rheolQgical properties [5,6]) is another significant factor. Figure 2 shows that thixotropic properties (in the acidic region) and distinct dilatant characteristics (in the alkaline region) become apparent depending on the pH value of the suspension. Figure 3 shows the results of a comparative study of the porosity of the castings obtained by slip casting and centrifugal casting from a given HCBS over a wide range of pH values. In contrast to the slip casting process during which a significant increase of Pcsg is observed in the region of low pH values (during thixotropic flow of the original HCBS), even a decrease of Pcsg can occur during the centrifugal casting process. On the other hand, in the pH range corresponding to dilatant flow, an increase of Pcsg is observed during centrifugal casting whereas in the case of slip casting, minimum values of Pcsg are observed in the same range.The aforementioned difference can be attributed to the 'deformation' characteristics of the pr...

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